Heat exchanger controls for low temperature fluids

• US 5,669,238 A
• Filed: 03/26/1996
• Issued: 09/23/1997
• Est. Priority Date: 03/26/1996
• Status: Expired due to Term

• Current Assignee: Conocophillips Company (ConocoPhillips)
• Original Assignee: Phillips Petroleum Company (ConocoPhillips)
• Inventors: Devers, Barnard J.
• Primary Examiner(s): KILNER, CHRISTOPHER B
• Application Number:
  US08/621,923
• Time in Patent Office: 546 Days
• Field of Search: 62/618, 62/657, 62/185
• US Class Current: 62/657
• CPC Class Codes: F25J 2200/02 in a single pressure main column system
• CPC Class Codes: F25J 2200/70 Refluxing the column with a condensed part of the feed stream, i.e. fractionator top is stripped or self-rectified
• CPC Class Codes: F25J 2210/06 Splitting of the feed stream, e.g. for treating or cooling in different ways
• CPC Class Codes: F25J 2220/60 Separating impurities from natural gas, e.g. mercury, cyclic hydrocarbons
• CPC Class Codes: F25J 2270/02 Internal refrigeration with liquid vaporising loop
• CPC Class Codes: F25J 2280/02 Control in general, load changes, different modes ("runs"), measurements
• CPC Class Codes: F25J 2280/10 Control for or during start-up and cooling down of the installation
• CPC Class Codes: F25J 3/0209 Natural gas or substitute natural gas
• CPC Class Codes: F25J 3/0233 separation of CnHm with 1 carbon atom or more
• CPC Class Codes: F25J 3/0238 separation of CnHm with 2 carbon atoms or more
• CPC Class Codes: F25J 3/0295 Start-up or control of the process; Details of the apparatus used, e.g. sieve plates, packings

Abstract

In a heat exchange scheme associated with a gas purification column in an LNG recovery process, in which heat exchange is desired between fluids of such widely different temperatures that thermal shock could result in damage to heat exchanger apparatus, a control scheme compensates for the effect of excessive temperature differential. The desired compensation is achieved by manipulating flow in a heat exchanger bypass conduit for the warm fluid to maintain a desired temperature ratio between the colder fluid entering the heat exchanger and the warmer fluid exiting the exchanger. Additionally, start-up controls for the column include temporarily selecting temperature of a cold stream to automatically control opening of a valve to initiate flow of the warm stream.

Claims

1. Apparatus comprising:

a) a cryogenic separation column for partially condensing a feed gas stream in an LNG recovery process;

b) means for withdrawing a liquid condensate stream from said cryogenic separation column;

c) a heat exchanger associated with said cryogenic separation column;

d) means for passing said liquid condensate stream through said heat exchanger;

e) means for passing a warm dry gas stream through said heat exchanger and thereafter to said cryogenic separation column, wherein said warm dry gas stream is cooled by indirect heat exchange with said liquid condensate stream in said heat exchanger;

f) a bypass conduit having a first control valve operably located therein for bypassing said warm dry gas stream around said heat exchanger;

g) means for establishing a first signal representative of the actual temperature of said warm dry gas stream exiting said heat exchanger;

h) means for establishing a second signal representative of the actual temperature of said liquid condensate stream entering said heat exchanger;

i) means for dividing said first signal by said second signal to establish a third signal representative of the ratio of said first signal and said second signal;

j) means for establishing a fourth signal representative of a desired value for the ratio represented by said third signal;

k) means for comparing said third signal and said fourth signal and establishing a fifth signal which is responsive to the difference of said third signal and said fourth signal, wherein said fifth signal is scaled to be representative of the position of said first control valve required to maintain the actual ratio represented by said third signal substantially equal to the desired ratio represented by said fourth signal; and

m) means for manipulating said first control valve in said bypass conduit in response to said fifth signal.

2. Apparatus in accordance with claim 1, additionally comprising:

means for establishing a sixth signal scaled to be representative of the flow rate of said liquid condensate stream required to maintain a desired liquid level in said cryogenic separation column; and

means for controlling the flow rate of said liquid condensate stream responsive to said sixth signal.

3. Apparatus in accordance with claim 2, additionally comprising:

a second control valve operably located so as to control flow of said warm dry gas stream; and

means for manipulating said second control valve responsive to a temperature selected from the pair of temperatures consisting of;

i. the actual temperature of said warm dry gas stream exiting said heat exchanger; and

ii. the actual temperature of said liquid condensate stream exiting said heat exchanger.

4. Apparatus in accordance with claim 3, wherein said means for manipulating said second control valve comprises:

means for establishing a seventh signal representative of the actual temperature of said liquid condensate stream exiting said heat exchanger;

means for establishing an eighth signal representative of the desired temperature of said liquid condensate stream exiting said heat exchanger;

means for comparing said seventh signal and said eighth signal to establish a ninth signal responsive to the difference of said seventh signal and said eighth signal, wherein said ninth signal is scaled to be representative of the position of said second control valve required to maintain the actual temperature of said liquid condensate stream exiting said heat exchanger represented by said seventh signal substantially equal to the desired temperature represented by said eighth signal;

means for establishing a tenth signal representative of the desired temperature of said warm dry gas stream exiting said heat exchanger represented by said second signal;

means for comparing said second signal and said tenth signal to establish an eleventh signal responsive to the difference between said second signal and said tenth signal, wherein said eleventh signal is scaled to be representative of the position of said second control valve required to maintain the actual temperature of said warm dry gas stream exiting said heat exchanger substantially equal to the desired value represented by said tenth signal;

means for establishing a twelfth signal selected as the one of said ninth signal and said eleventh signal having the higher value; and

means for manipulating said second control valve responsive to said twelfth signal.

5. A method for controlling temperature in a heat exchanger equipped with a bypass conduit having a first control valve operatively connected therein, said heat exchanger being associated with a cryogenic separation column that removes a benzene contaminant from a feed stream in and LNG recovery process, said method comprising:

withdrawing a liquid condensate stream at a cryogenic temperature from said cryogenic separation column;

passing said liquid condensate stream through said heat exchanger;

passing a warm dry gas stream through said heat exchanger and thereafter introducing said warm dry gas stream into said cryogenic separation column, wherein said warm dry gas stream is cooled by indirect heat exchange with said liquid condensate stream in said heat exchanger;

establishing a first signal representative of the actual temperature of said warm dry gas stream exiting said heat exchanger;

establishing a second signal representative of the actual temperature of said liquid condensate stream entering said heat exchanger;

dividing said first signal by said second signal to establish a third signal representative of the ratio of said first signal and said second signal;

establishing a fourth signal representative of a desired value for said third signal;

comparing said third signal and said fourth signal and establishing a fifth signal which is responsive to the difference between said third signal and said fourth signal, wherein said fifth signal is scaled to be representative of the position of said first control valve required to maintain the actual ratio represented by said third signal substantially equal to the desired ratio represented by said fourth signal; and

manipulating said first control valve in said bypass conduit in response to said fifth signal.

6. A method in accordance with claim 5 additionally comprising the following steps:

establishing a sixth signal scaled to be representative of the flow rate of said liquid condensate steam required to maintain a desired liquid level in said cryogenic separation column; and

controlling the flow rate of said liquid condensate stream responsive to said sixth signal.

7. A method in accordance with claim 6, wherein a second control valve is operably located so as to control flow rate of said warm dry gas stream, said method additionally comprising the following steps:

manipulating said second control valve responsive to a temperature selected from the pair of temperatures consisting of;

i) the actual temperature of said warm dry gas stream exiting said heat exchanger; and

ii) the actual temperature of said liquid condensate stream exiting said heat exchanger.

8. A method in accordance with claim 7, wherein said step of manipulating said second control valve comprises:

establishing a seventh signal representative of the actual temperature of said liquid condensate stream exiting said heat exchanger;

establishing an eighth signal representative of the desired temperature of said liquid condensate stream exiting said heat exchanger;

comparing said seventh signal and said eighth signal to establish a ninth signal responsive to the difference between said seventh signal and said eighth signal, wherein said ninth signal is scaled to be representative of the position of said second control valve required to maintain the actual temperature of said liquid condensate stream exiting said heat exchanger represented by said seventh signal substantially equal to the desired temperature represented by said eighth signal;

establishing a tenth signal representative of the desired temperature of said warm dry gas stream exiting said heat exchanger represented by said second signal;

comparing said second signal and said tenth signal to establish an eleventh signal responsive to the difference between said second signal and said tenth signal, wherein said eleventh signal is scaled to be representative of the position of said second control valve required to maintain the actual temperature of said warm dry gas stream exiting said heat exchanger substantially equal to the desired value represented by said tenth signal;

establishing a twelfth signal selected as the one of said ninth signal and said eleventh signal having the higher value; and

manipulating said second control valve responsive to said twelfth signal.